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Comment by sandworm101 | Hacker News Reader

Who is telling you that the universe is infinite? Astronomers/cosmologists generally all agree that our current universe (the matter-containing 4 dimensions bit we live in) had a defined beginning from which is has grown. There is an outer edge,. defined by the rate of expansion starting from the big bang.

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What you're describing is the 'visible universe'. There may well be other parts of the cosmos outside of our light cone. Einstein's equations allow it, and we have no way of knowing.

No, visible universe is what we can see, it's defined as all the points close enough in spacetime that light from there had enough time to get to us. Visible universe is centered on Earth, much smaller than what GP was talking about, and decreasing steadily (because at the edge of it expansion of space pushes stuff outside faster than speed of light).

at the edge of it expansion of space pushes stuff outside faster than speed of light

Not quite: According to the cosmological standard model, the visible universe will continue to grow (ie new galaxies will continue to come into view) - but only asymptotically, ie until a maximum size given by the comological event horizon is reached. However, the parts of the universe that aren't gravitationally bound to us will become fainter and fainter and increasingly resdhifted, and eventually, we'll be unable to detect other theoretically visible galaxies due to technological limitations.

If they are moving away from us faster than speed of light because of expansion of space - wouldn't it mean at some point no new light from them reaches us? Even ignoring the limitation of equipment?

In my opinion, that whole 'moving away from us faster than c' business is not really a good way to think about this: For one, we can see to a redshift of about 10, corresponding to a comoving distance of about 30Gly, and a recession velocity of about 4c (four times the speed of light!) at time of emission.

There's a cosmological event horizon. Light emitted from within will reach us in finite time, light emitted from without won't. Similar to how a distant observer will never see on object falling into a (stationary) black hole cross the Schwarzschild horizon, we won't see galaxies crossing the cosmological horizon.

The Big Bang supposedly encompassed all of space and had no edges.

Has no edges, and therefore unbounded, but still finite in size.

How can it be both finite in size, and have no edges?

To me, it would seem that were it finite, there would be a point at which one would look back, and see the galaxy and clusters that compose the universe; forward would be an expanse of nothingness. But if this isn't the case, then how I can keep progressing forward (presumably forever, as I can't hit an edge) through space, encountering galaxy after galaxy, but it is still finite?

Unless this is like RPG games where the edges wrap.

How can it be both finite in size, and have no edges?

Good question, but find me the edge of an idealized balloon. Where is the edge of a sphere? As to why you won’t come back, remember that spacetime is expanding faster an faster, and you can only travel below light speed. Mind you that’s just one possibility. The universe at large could be a lot of different things, but as humans were causally disconnected from anything beyond the shrinking observable universe. Shrinking from our perspective at least, because of the aforementioned expansion and speed limit.

It is exactly the case that the edges wrap (presuming that I understand correctly, of course).

It is a running theory that space time could be curved and wrap, or be more traditionally flat, or even be some kind of saddle that means it is still curved but never meets itself again like a sphere does. Recent studies point to it being flat though, if I remember correctly.

The surface of the Earth is finite in size and has no edges. You can keep progressing forward without hitting an edge.

There have been recent studies which show that universe is probably "flat" though.

Or very slightly curved - measurements come with error bars, so we can't be sure if it's exactly flat. Also note that flatness and finite size are compatible in case of non-trivial topology (think of the flat torus - pacman world - which, however, is not isotropic).

All "space" ie the matter-containing 4d part we live in. There are things before and perhaps outside our universe, brains and such, but that is outside "our universe".

But what if your Big Bang was only a local bang among an infinite number of bangs so far apart from one another that the light from any of them would take 100s of billions of your earth years to reach its nearest neighbor. Then, even now, they are all expanding toward each other with no way for any of their passengers to know it. You won't need to worry about a heat death or a cold death, you will have a death by collision and probably some kind of rebirth to follow.

Then what do physicists call what is beyond the edge of the universe?

edit: excuse my french ;)

It may mean beyond the observable universe.

The universe may or may not be infinite, for some particular definition of infinity. Our light cone, which essentially defines our universe from our point of view, is distinct and measurably finite.

I don't think my physician calls the edge of the universe anything unless he's a hobbyist physicist as well.

Imagine you are in inside of a giant beach ball, walking on the surface. You will never find an edge, and it is meaningless to talk about such a thing. Yet the space is finite.

Conversely, the surface of the beach ball has no "center". It's also fun to realize if you blow that beach ball up, from the point of view of any point on the ball, all other points grow further away.

A beach ball has no "edge", but it definitely has a boundary surface, and everything on the other side of this surface is "not beach ball".

Now what lies beyond the universe's boundary surface? Is such a surface even present?

That is indeed where the analogy starts to crumble. The reason is that space itself is curved. It is curved by the matter inside it. Still the property holds that if you travelled far enough fast enough you’d come back to where you started.

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